Method for producing monocyclic aromatic hydrocarbons

ABSTRACT

A method for producing monocyclic aromatic hydrocarbons includes a step of introducing a feedstock oil into a cracking/reforming reactor, bringing the feedstock oil into contact with a catalyst, and causing the feedstock oil to react, a step of purifying and recovering the monocyclic aromatic hydrocarbons separated from the product produced in the reaction step, a step of hydrogenating a heavy fraction separated from the product, and a recycling step of returning a hydrogenation reactant of the heavy fraction to the cracking/reforming reaction step. In the recycling step, the hydrogenation reactant is introduced at a location different from an introduction location of the feedstock oil into the reactor so that a time during which the hydrogenation reactant is in contact with the catalyst in the reactor becomes shorter than a time during which the feedstock oil is in contact with the catalyst in the reactor.

TECHNICAL FIELD

The present invention relates to a method for producing monocyclic aromatic hydrocarbons from oil containing a large amount of polycyclic aromatic hydrocarbons.

Priority is claimed on Japanese Patent Application No. 2011-067692, filed on Mar. 25, 2011, the content of which is incorporated herein by reference.

BACKGROUND ART

Light cycle oil (hereinafter, referred to as “LCO”), which is cracked light oil produced using a fluid catalytic cracking apparatus, contains a large amount of polycyclic aromatic hydrocarbons, and has been utilized as light oil or heavy oil. However, in recent years, a method has been proposed to obtain monocyclic aromatic hydrocarbons with a high added value (for example, benzene, toluene, xylene, and ethylbenzene) that can be utilized as high octane gasoline base materials or petroleum chemistry raw materials, from the LCO (for example, refer to Patent Document 1).

PRIOR ART DOCUMENTS Patent Documents

-   [Patent Document 1] PCT International Publication No. WO 2010/109899

DISCLOSURE OF INVENTION Technical Problem

However, in the method described in Patent Document 1, it cannot be said that the yield of monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms is sufficiently high at all times.

In addition, in plants for producing the monocyclic aromatic hydrocarbons, there is a general demand for the miniaturization of reactors and the like in order to reduce apparatus costs or operation costs.

An object of the invention is to provide a method for producing monocyclic aromatic hydrocarbons which can produce monocyclic hydrocarbons having 6 to 8 carbon atoms with a high yield from a feedstock oil containing polycyclic aromatic hydrocarbons, and also can miniaturize cracking/reforming reactors.

Solution to Problem

The present inventors repeated thorough studies in order to achieve the above object, and obtained the following knowledge.

Monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms can be produced with a high yield by subjecting a feedstock oil to a cracking/reforming reaction step, hydrogenating a heavy fraction having 9 or more carbon atoms which is separated from a produced product, and then returning a hydrogenation reactant of the obtained heavy fraction to the cracking/reforming reaction step.

Therefore, the inventors repeated studies based on the above knowledge in order to further improve the production steps and the like, and, consequently, completed the invention.

That is, a method for producing monocyclic aromatic hydrocarbons according to a first aspect of the invention is a method for producing monocyclic aromatic hydrocarbons, by which monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms are produced from a feedstock oil having a 10 vol % distillation temperature of 140° C. or higher and a 90 vol % distillation temperature of 380° C. or lower, including:

a cracking/reforming reaction step of introducing the feedstock oil into a cracking/reforming reactor loaded with a catalyst for monocyclic aromatic hydrocarbon production containing a crystalline aluminosilicate, bringing the feedstock oil into contact with the catalyst for monocyclic aromatic hydrocarbon production, and causing the feedstock oil to react, thereby obtaining a product containing monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms;

a purification/recovery step of purifying and recovering the monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms separated from the product produced in the cracking/reforming reaction step;

a hydrogenation reaction step of hydrogenating a heavy fraction having 9 or more carbon atoms separated from the product produced in the cracking/reforming reaction step; and

a recycling step of returning a hydrogenation reactant of the heavy fraction obtained in the hydrogenation reaction step to the cracking/reforming reaction step,

in which, in the recycling step, the hydrogenation reactant of the heavy fraction is introduced at a location different from an introduction location of the feedstock oil into the cracking/reforming reactor so that a time during which the hydrogenation reactant of the heavy fraction is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor becomes shorter than a time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor.

A method for producing monocyclic aromatic hydrocarbons according to a second aspect of the invention is a method for producing monocyclic aromatic hydrocarbons, by which monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms are produced from a feedstock oil having a 10 vol % distillation temperature of 140° C. or higher and a 90 vol % distillation temperature of 380° C. or lower, including:

a cracking/reforming reaction step of introducing the feedstock oil into a cracking/reforming reactor loaded with a catalyst for monocyclic aromatic hydrocarbon production containing a crystalline aluminosilicate, bringing the feedstock oil into contact with the catalyst for monocyclic aromatic hydrocarbon production, and causing the feedstock oil to react, thereby obtaining a product containing monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms;

a hydrogenation reaction step of hydrogenating a portion of the product produced in the cracking/reforming reaction step;

a purification/recovery step of distilling a hydrogenation reactant obtained in the hydrogenation reaction step so as to purify and collect monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms; and

a recycling step of returning the hydrogenation reactant of the heavy fraction having 9 or more carbon atoms separated and removed from the monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms in the purification/recovery step,

in which, in the recycling step, the heavy fraction is introduced at a location different from an introduction location of the feedstock oil into the cracking/reforming reactor so that a time during which the hydrogenation reactant of the heavy fraction is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor becomes shorter than a time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor.

In addition, in the method for producing monocyclic aromatic hydrocarbons, in the recycling step, when the time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor is considered to be 100%, the hydrogenation reactant of the heavy fraction or the heavy fraction is preferably introduced at a location different from the introduction location of the feedstock oil into the cracking/reforming reactor so that the time during which the hydrogenation reactant of the heavy fraction or the heavy fraction is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor becomes 10% to 80%.

In addition, in the method for producing monocyclic aromatic hydrocarbons, in a case in which the cracking/reforming reactor employs a fluidized bed type and a single-stage configuration, the hydrogenation reactant of the heavy fraction or the heavy fraction is preferably introduced on a downstream side of the introduction location into the cracking/reforming reactor.

In addition, in the method for producing monocyclic aromatic hydrocarbons, in a case in which the cracking/reforming reactor employs a fixed bed type and a multistage configuration, it is preferable that the feedstock oil be introduced into a first reactor in the cracking/reforming reactor, and the hydrogenation reactant of the heavy fraction or the heavy fraction be introduced into a reactor behind the first reactor.

In addition, in the method for producing monocyclic aromatic hydrocarbons, the crystalline aluminosilicate contained in the catalyst for monocyclic aromatic hydrocarbon production used in the cracking/reforming reaction step preferably contains a medium-pore zeolite and/or a large-pore zeolite as a main component.

In addition, the method for producing monocyclic aromatic hydrocarbons preferably includes a hydrogen recovery step of recovering hydrogen produced as a by-product in the cracking/reforming reaction step from a product obtained in the cracking/reforming reaction step and a hydrogen supply step of supplying the hydrogen collected in the hydrogen recovery step to the hydrogenation reaction step.

Advantageous Effects of Invention

According to the method for producing monocyclic aromatic hydrocarbons of the invention, monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms can be produced from a feedstock oil containing polycyclic aromatic hydrocarbons with a high yield, and cracking/reforming reactors can also be miniaturized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic configuration view of a production plant according to Embodiment 1 of the method for producing monocyclic aromatic hydrocarbons of the invention.

FIG. 2 is a schematic configuration view of a production plant according to Embodiment 2 of the method for producing monocyclic aromatic hydrocarbons of the invention.

FIG. 3 is a schematic configuration view of a production plant according to Embodiment 3 of the method for producing monocyclic aromatic hydrocarbons of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION Embodiment 1

Embodiment 1 of the method for producing monocyclic aromatic hydrocarbons of the invention will be described.

The method for producing monocyclic aromatic hydrocarbons of the present Embodiment is a method for producing monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms from a feedstock oil (hereinafter, referred to as “monocyclic aromatic hydrocarbons”) including the following (a) to (h) steps. In addition, FIG. 1 is a schematic configuration view of a production plant for describing Embodiment 1.

(a) A cracking/reforming reaction step of introducing a feedstock oil into a cracking/reforming reactor 10 loaded with a catalyst for monocyclic aromatic hydrocarbon production, and causing the raw material to react, thereby obtaining a product containing monocyclic aromatic hydrocarbons;

(b) a separation step of separating the product produced in the cracking/reforming reaction step (cracking/reforming reactor 10) into plural fractions using a separation apparatus 11;

(c) a purification/recovery step of purifying and recovering the monocyclic aromatic hydrocarbons separated in the separation step (separation apparatus 11) using a purification/recovery apparatus 12;

(d) a heavy fraction discharge step of discharging a portion of the heavy fraction having 9 or more carbon atoms (hereinafter, shortened as “heavy fraction”) obtained from the fraction separated in the separation step (separation apparatus 11) out of a system;

(e) a hydrogenation reaction step of hydrogenating a heavy fraction not discharged out of the system in the heavy fraction discharge step using a hydrogenation reactor 13;

(f) a hydrogen recovery step of recovering hydrogen produced as a by-product in the cracking/reforming reaction step from a gas component separated in the separation step (separation apparatus 11) using a hydrogen recovery apparatus 14;

(g) a hydrogen supply step of supplying the hydrogen collected in the hydrogen recovery step (hydrogen recovery apparatus 14) to the hydrogenation reaction step (hydrogenation reactor 13); and

(h) a recycling step of introducing (returning) a hydrogenation reactant of the heavy fraction obtained in the hydrogenation reaction step (hydrogenation reactor 13) at a location different from an introduction location of the feedstock oil into the cracking/reforming reactor 10 (cracking/reforming reaction step).

Among the steps (a) to (h), the steps (a), (c), (e), and (f) are essential steps in the invention according to claim 1, and the steps (b), (d), (f), and (g) are optional steps.

Hereinafter, the respective steps will be described in detail.

<Cracking/Reforming Reaction Step>

In the cracking/reforming reaction step, a feedstock oil is introduced into the cracking/reforming reactor 10 loaded with a catalyst for monocyclic aromatic hydrocarbon production, brought into contact with the catalyst for monocyclic aromatic hydrocarbon production, and made to react. Thereby, saturated hydrocarbons contained in the feedstock oil are used as a hydrogen-donating source, polycyclic aromatic hydrocarbons are partially hydrogenated by a hydrogen transfer reaction from the saturated hydrocarbons, and ring-opening is carried out, whereby the polycyclic aromatic hydrocarbons are converted to monocyclic aromatic hydrocarbons. Furthermore, the saturated hydrocarbons that are present in the feedstock oil or are obtainable in a separation operation can also be converted to monocyclic aromatic hydrocarbons through cyclization and dehydrogenation. Also, monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms can also be obtained by cracking monocyclic aromatic hydrocarbons having 9 or more carbon atoms. Meanwhile, a product contains hydrogen, methane, ethane, LPG, a heavy fraction having 9 or more carbon atoms and the like in addition to the monocyclic aromatic hydrocarbons.

(Feedstock Oil)

The feedstock oil used in the present Embodiment is an oil having a 10 vol % distillation temperature of 140° C. or higher and a 90 vol % distillation temperature of 380° C. or lower. Regarding an oil having a 10 vol % distillation temperature of lower than 140° C., since the oil is light oil, monocyclic aromatic hydrocarbons are produced, so that the oil does not suit the purpose of the present Embodiment. Furthermore, in the case of using an oil having a 90 vol % distillation temperature of higher than 380° C., the yield of monocyclic aromatic hydrocarbons is decreased, the amount of coke deposition on the catalyst for monocyclic aromatic hydrocarbon production increases, and a rapid decrease in the catalyst activity tends to occur.

The 10 vol % distillation temperature of the feedstock oil is preferably 150° C. or higher, and the 90 vol % distillation temperature of the feedstock oil is preferably 360° C. or lower.

In addition, the 10 vol % distillation temperature and the 90 vol % distillation temperature as used herein mean values measured according to JIS K2254 “Petroleum products—Distillation Testing Methods.”

Examples of feedstock oils having a 10 vol % distillation temperature of 140° C. or higher and a 90 vol % distillation temperature of 380° C. or lower include cracked light oil produced using a fluid catalytic cracking apparatus (LCO), hydrogenation purified oil of LCO, coal liquefaction oil, heavy oil hydrocracking purified oil, straight run kerosene, straight run gas oil, coker kerosene, coker gas oil, and oil sand hydrocracking purified oil.

Polycyclic aromatic hydrocarbons are materials that have low reactivity and are not easily converted to monocyclic aromatic hydrocarbons in the cracking/reforming reaction step of the present Embodiment. However, on the other hand, when hydrogenated in the hydrogenation reaction step, polycyclic aromatic hydrocarbons are converted to naphthenobenzenes, and can be further converted to monocyclic aromatic hydrocarbons by subsequently being supplied to be recycled to the cracking/reforming reaction step. However, among the polycyclic aromatic hydrocarbons, aromatic hydrocarbons having three or more rings consume a large amount of hydrogen in the hydrogenation reaction step, and even though those hydrocarbons are hydrogenation reactants, since the reactivity in the cracking/reforming reaction step is low, it is not preferable for the feedstock oil to contain a large amount of the polycyclic aromatic hydrocarbons. Therefore, the content of aromatic hydrocarbons having three or more rings in the feedstock oil is preferably 25 vol % or less, and more preferably 15 vol % or less.

In addition, regarding the feedstock oil containing bicyclic aromatic hydrocarbons that are converted to naphthenobenzene in the hydrogenation reaction step and intended to reduce aromatic hydrocarbons having three or more rings, for example, it is more preferable that the 90 vol % distillation temperature of the feedstock oil be 330° C. or lower.

Furthermore, the polycyclic aromatic hydrocarbons as used herein mean the total value of the content of bicyclic aromatic hydrocarbons (bicyclic aromatic fraction) and the content of aromatic hydrocarbons having three or more rings (tricyclic or higher-cyclic aromatic fraction) that are measured according to JPI-5S-49 “Petroleum products—Hydrocarbon type test methods—High performance liquid chromatography method”, or analyzed by an FID gas chromatographic method (FID: Flame ionization detector) or a two-dimensional gas chromatographic method. Hereinafter, when the contents of polycyclic aromatic hydrocarbons, bicyclic aromatic hydrocarbons, and aromatic hydrocarbons having three or more rings are expressed in vol %, the contents are values measured according to JPI-5S-49, and when the contents are expressed in mass %, the values are measured based on an FID gas chromatographic method or a two-dimensional gas chromatographic method.

(Reaction Type)

Regarding the reaction type at the time of bringing the feedstock oil into contact with the catalyst for monocyclic aromatic hydrocarbon production and causing the feedstock oil to react, that is, the reaction type of the cracking/reforming reactor 10, examples thereof include a fixed bed, a moving bed, and a fluidized bed.

In the present Embodiment, since a heavy fraction is used as the raw material, a fluidized bed in which a coke fraction adhered to the catalyst can be continuously removed and the reaction can be carried out in a stable manner is preferred, and a continuous regeneration type fluidized bed in which a catalyst is circulated between a reactor and a regenerator and thus reaction and regeneration can be continuously repeated is particularly preferred. In addition, as the reaction type, riser cracking in which a contact reaction is carried out in a tube through which a raw material and a catalyst move together, bed cracking in which a reaction is carried out in a fluidizing catalyst layer, and the like are known. Among the reaction types, since the apparatus configuration can be more simplified in the bed cracking than in the riser cracking, the bed cracking type is more preferably employed in the cracking/reforming reactor (cracking/reforming reaction step) of the present Embodiment. Meanwhile, in other Embodiments that will be described below, an example in which a fixed bed cracking/reforming reactor is used will be described.

The feedstock oil at the time of bringing the catalyst for monocyclic aromatic hydrocarbon production into contact is preferably in a gaseous state. Furthermore, the raw material may be diluted by means of a gas as necessary.

(Catalyst for Monocyclic Aromatic Hydrocarbon Production)

The catalyst for monocyclic aromatic hydrocarbon production contains a crystalline aluminosilicate.

[Crystalline Aluminosilicate]

The crystalline aluminosilicate is preferably a medium-pore zeolite and/or a large-pore zeolite from the viewpoint that the yield of monocyclic aromatic hydrocarbons can be further increased.

A medium-pore zeolite is a zeolite having a 10-membered ring skeletal structure, and examples of the medium-pore zeolite include zeolites having crystal structures of AEL type, EUO type, FER type, HEU type, MEL type, MFI type, NES type, TON type, and WEI type. Among these, from the viewpoint of further increasing the yield of monocyclic aromatic hydrocarbons, MFI type is preferred.

A large-pore zeolite is a zeolite having a 12-membered ring skeletal structure, and examples of the large-pore zeolite include zeolites having crystal structures of AFI type, ATO type, BEA type, CON type, FAU type, GME type, LTL type, MOR type, MTW type, and OFF type. Among these, from the viewpoint of being industrially usable, zeolites of BEA type, FAU type and MOR type are preferred, and from the viewpoint of further increasing the yield of monocyclic aromatic hydrocarbons, a zeolite of BEA type is preferred.

The crystalline aluminosilicate may contain a small-pore zeolite having a 10-membered or fewer-membered ring skeletal structure, or an ultralarge-pore zeolite having a 14-membered or more-membered ring skeletal structure, in addition to the medium-pore zeolite and the large-pore zeolite.

Here, examples of the small-pore zeolite include zeolites having crystal structures of ANA type, CHA type, ERI type, GIS type, KFI type, LTA type, NAT type, PAU type, and YUG type.

Examples of the ultralarge-pore zeolite include zeolites having crystal structures of CLO type and VPI type.

In the present Embodiment, since the cracking/reforming reaction step is carried out by a fluidized bed reaction, the content of the crystalline aluminosilicate in the catalyst for monocyclic aromatic hydrocarbon production is preferably 20 mass % to 60 mass %, more preferably 30 mass % to 60 mass %, and particularly preferably 35 mass % to 60 mass %, when the total amount of the catalyst for monocyclic aromatic hydrocarbon production is designated as 100 mass %. If the content of the crystalline aluminosilicate is 20 mass % or more, the yield of monocyclic aromatic hydrocarbons can be sufficiently increased. If the content of the crystalline aluminosilicate is more than 60 mass %, the content of the binder that can be incorporated into the catalyst is reduced, and the catalyst may become unsuitable for fluidized bed applications.

Meanwhile, as described in an Embodiment that will be described below, when the cracking/reforming reaction step is carried out by a fixed bed reaction, the content of the crystalline aluminosilicate in the catalyst for monocyclic aromatic hydrocarbon production is preferably 60 mass % to 100 mass %, more preferably 70 mass % to 100 mass %, and particularly preferably 90 mass % to 100 mass %, when the total amount of the catalyst for monocyclic aromatic hydrocarbon production is designated as 100 mass %. If the content of the crystalline aluminosilicate is 60 mass % or more, the yield of monocyclic aromatic hydrocarbons can be sufficiently increased.

[Phosphorus and Boron]

For the catalyst for monocyclic aromatic hydrocarbon production, it is preferable that the catalyst contain phosphorus and/or boron. When the catalyst for monocyclic aromatic hydrocarbon production contains phosphorus and/or boron, a decrease over time in the yield of the monocyclic aromatic hydrocarbons can be prevented, and coke production at the catalyst surface can be suppressed.

Examples of the method for incorporating phosphorus into the catalyst for monocyclic aromatic hydrocarbon production include a method of supporting phosphorus on a crystalline aluminosilicate, a crystalline aluminogallosilicate, or a crystalline aluminozincosilicate by means of an ion exchange method, an impregnation method or the like; a method of incorporating a phosphorus compound at the time of zeolite synthesis, thereby substituting a portion in the skeleton of a crystalline aluminosilicate with phosphorus; and a method of using a crystallization accelerator containing phosphorus at the time of zeolite synthesis. The phosphate ion-containing aqueous solution to be used at that time is not particularly limited, but aqueous solutions prepared by dissolving phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and other water-soluble phosphoric acid salts in water at arbitrary concentrations can be preferably used.

Examples of the method for incorporating boron into the catalyst for monocyclic aromatic hydrocarbon production include a method of supporting boron on a crystalline aluminosilicate, a crystalline aluminogallosilicate, or a crystalline aluminozincosilicate by means of an ion exchange method, an impregnation method or the like; a method of incorporating a boron compound at the time of zeolite synthesis and thereby substituting a portion in the skeleton of a crystalline aluminosilicate with boron; and a method of using a crystallization accelerator containing boron at the time of zeolite synthesis.

The content of phosphorus and boron in the catalyst for monocyclic aromatic hydrocarbon production is preferably 0.1 mass % to 10 mass %, furthermore, the lower limit is more preferably 0.5 mass % or more, the upper limit is more preferably 9 mass % or less, and particularly preferably 8 mass % or less with respect to the total mass of the catalyst. When the content of phosphorous is 0.1 mass % with respect to the total mass of the catalyst, a decrease over time in the yield of the monocyclic aromatic hydrocarbons can be prevented, and, when the content of phosphorous is 10 mass % or less, the yield of monocyclic aromatic hydrocarbons can be increased.

[Gallium and Zinc]

The catalyst for monocyclic aromatic hydrocarbon production can contain gallium and/or zinc as necessary. When gallium and/or zinc is incorporated, the production proportion of the monocyclic aromatic hydrocarbons can be further increased.

Examples of the form of gallium incorporation in the catalyst for monocyclic aromatic hydrocarbon production include a form in which gallium is incorporated into the lattice skeleton of the crystalline aluminosilicate (crystalline aluminogallosilicate), a form in which gallium is supported on the crystalline aluminosilicate (gallium-supported crystalline aluminosilicate), and a form including both.

Examples of the form of zinc incorporation in the catalyst for monocyclic aromatic hydrocarbon production include a form in which zinc is incorporated into the lattice skeleton of the crystalline aluminosilicate (crystalline aluminozincosilicate), a form in which zinc is supported in the crystalline aluminosilicate (zinc-supported crystalline aluminosilicate), and a form including both.

A crystalline aluminogallosilicate and a crystalline aluminozincosilicate have a structure in which SiO₄, AlO₄ and GaO₄ structures exist in the skeleton. Furthermore, the crystalline aluminogallosilicate and crystalline aluminozincosilicate are obtained by, for example, gel crystallization based on hydrothermal synthesis, a method of inserting gallium or zinc into the lattice skeleton of a crystalline aluminosilicate, or a method of inserting aluminum into the lattice skeleton of a crystalline gallosilicate or a crystalline zincosilicate.

A gallium-supported crystalline aluminosilicate is a material in which gallium is supported on a crystalline aluminosilicate according to a known method such as an ion exchange method or an impregnation method. The gallium source used at that time is not particularly limited, but examples thereof include gallium salts such as gallium nitrate and gallium chloride, and gallium oxide.

A zinc-supported crystalline aluminosilicate is a material in which zinc is supported on a crystalline aluminosilicate according to a known method such as an ion exchange method or an impregnation method. The zinc source used at that time is not particularly limited, but examples thereof include zinc salts such as zinc nitrate and zinc chloride, and zinc oxide.

When the catalyst for monocyclic aromatic hydrocarbon production contains gallium and/or zinc, the content of gallium and/or zinc in the catalyst for monocyclic aromatic hydrocarbon production is preferably 0.01 mass % to 5.0 mass %, and more preferably 0.05 mass % to 2.0 mass %, when the total amount of the catalyst is designated as 100 mass %. If the content of gallium and/or zinc is 0.01 mass % or more, the production proportion of the monocyclic aromatic hydrocarbons can be further increased, and if the content is 5.0 mass % or less, the yield of the monocyclic aromatic hydrocarbons can be further increased.

[Shape]

The catalyst for monocyclic aromatic hydrocarbon production is formed into, for example, a powder form, a particulate form, or a pellet form, depending on the reaction type. For example, in the case of a fluidized bed as in the present Embodiment, the catalyst is formed in a powder form, and in the case of a fixed bed as in other Embodiments, the catalyst is formed into a particulate form or a pellet form. The average particle size of the catalyst used in a fluidized bed is preferably 30 μm to 180 μm, and more preferably 50 μm to 100 μm. Furthermore, the apparent density of the catalyst used in a fluidized bed is preferably 0.4 g/cc to 1.8 g/cc, and more preferably 0.5 g/cc to 1.0 g/cc.

In addition, the average particle size represents a particle size which corresponds to 50 mass % in a particle size distribution obtained by classification with sieves, and the apparent density is a value measured by the method of JIS Standard R9301-2-3.

In the case of obtaining a particulate or pellet-shaped catalyst, according to necessity, an inert oxide is incorporated into the catalyst as a binder, and then the blend may be molded using various molding machines.

When the catalyst for monocyclic aromatic hydrocarbon production contains an inorganic oxide such as a binder, a binder containing phosphorus may be used without any problem.

(Reaction Temperature)

The reaction temperature at the time of bringing feedstock oil into contact with a catalyst for monocyclic aromatic hydrocarbon production to react is not particularly limited, but the reaction temperature is preferably set to 400° C. to 650° C. If the lower limit of the reaction temperature is 400° C. or higher, the feedstock oil can be made to react easily, and the lower limit is more preferably 450° C. or higher. Furthermore, if the upper limit of the reaction temperature is 650° C. or lower, the yield of the monocyclic aromatic hydrocarbons can be sufficiently increased, and the upper limit is more preferably 600° C. or lower.

(Reaction Pressure)

The reaction pressure at the time of bringing feedstock oil and recycle oil that will be described below into contact with a catalyst for monocyclic aromatic hydrocarbon production and causing a reaction is preferably set to 1.5 MPaG or less, and more preferably set to 1.0 MPaG or less. If the reaction pressure is 1.5 MPaG or less, production of by-products of light gas can be suppressed, and also, pressure resistance of the reaction apparatus can be lowered.

(Contact Time)

The contact time for the feedstock oil and the catalyst for monocyclic aromatic hydrocarbon production is not particularly limited as long as a desired reaction substantially proceeds; however, for example, the contact time as the time for gas passage on the catalyst for monocyclic aromatic hydrocarbon production is preferably 1 second to 300 seconds, and it is more preferable to set the lower limit to 5 seconds or longer and the upper limit to 150 seconds or shorter. If the contact time is 1 second or longer, the reaction can be carried out reliably, and if the contact time is 300 seconds or shorter, accumulation of carbonaceous materials on the catalyst due to coking or the like can be suppressed. Also, the amount of generation of light gas due to cracking can be suppressed.

In the present Embodiment, the introduction location of feedstock oil into the cracking/reforming reactor 10 is set at the bottom portion of the reactor 10 so that the time during which the feedstock oil comes into contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10, that is, the time for gas passage on the catalyst for monocyclic aromatic hydrocarbon production becomes, for example, 1 second to 300 seconds.

Meanwhile, the contact time between the recycle oil, that is, the hydrogenation reactant of the heavy fraction obtained using the hydrogenation reactor 13 (hydrogenation reaction step) in FIG. 1 and the catalyst for monocyclic aromatic hydrocarbon production is set to be shorter than the contact time between the feedstock oil and the catalyst for monocyclic aromatic hydrocarbon production. That is, the time during which the recycle oil (the hydrogenation reactant of the heavy fraction) is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 is set to be shorter than the time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10.

This is because monocyclic aromatic hydrocarbons in the heavy fraction are partially hydrogenated due to the hydrogenation reaction so as to be converted to naphthenobenzene, and therefore the hydrogenation reactant of the heavy fraction obtained using the hydrogenation reactor 13 (hydrogenation reaction step) obtains an increased reactivity compared with the feedstock oil, and can produce a sufficient amount of monocyclic aromatic hydrocarbons even when the contact time with the catalyst is shorter.

Conversely, if the contact time (reaction time) is set to be equal to the contact time of the feedstock oil, the proportion of over-cracked recycle oil which has a high reactivity increases such that the production amount of gas increases. Meanwhile, when the contact time of the feedstock oil is decreased in order to suppress an increase in the gas production amount, there is a concern that the cracking reaction of the feedstock oil may not sufficiently proceed.

Therefore, in the present Embodiment, the contact time with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 is set to be shorter for the recycle oil than for the feedstock oil. Specifically, if the contact time with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 is designated as 100%, the time during which the recycle oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 is preferably set to 10% to 80%. When the time becomes less than 10%, the contact time of the recycle oil with the catalyst for monocyclic aromatic hydrocarbon production becomes short, and there is a concern that the production of monocyclic aromatic hydrocarbons from the recycle oil may become insufficient. On the other hand, when the time is more than 80%, there is a concern that the effect of the shorter contact time for the recycle oil than for the feedstock oil may not be sufficiently obtained. The time during which the recycle oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 is more preferably 15% to 70%.

That is, if the contact time of the recycle oil with the catalyst in the cracking/reforming reactor 10 is set to be shorter than the contact time of the feedstock oil with the catalyst, the retention time of the recycle oil in the cracking/reforming reactor 10 inevitably becomes short. The necessary volume of the cracking/reforming reactor 10 for design is mainly determined by the loading amount of the catalyst for monocyclic aromatic hydrocarbon production and the retention amounts of the feedstock oil and the recycle oil in the cracking/reforming reactor 10.

The retention amount of the feedstock oil or the recycle oil is determined by the product of the amount of the feedstock oil or the recycle oil introduced into the cracking/reforming reactor 10 per unit time and the retention time. Therefore, the retention amount of the recycle oil can be decreased by the decreased retention time of the recycle oil compared with the retention time of the feedstock oil compared with a case in which the retention time is equal to the retention time of the feedstock oil.

That is, the amount of the feedstock oil introduced into the cracking/reforming reactor 10 per unit time and the amount of the recycle oil per unit time are respectively set to be constant, in the present Embodiment, the volume of the cracking/reforming reactor 10 can be decreased by the decreased retention time of the recycle oil compared with the retention time of the feedstock oil compared with a case in which the retention times are set to be equal. Therefore, it is possible to miniaturize the cracking/reforming reactor 10.

However, as described above, if the time during which the recycle oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 is more than 80% of the time for the feedstock oil, it becomes difficult to miniaturize the cracking/reforming reactor 10, and the effect of the miniaturization for the reduction of the apparatus costs becomes small.

In order to set the contact time of the recycle oil with the catalyst in the cracking/reforming reactor 10 to be shorter than the contact time of the feedstock oil with the catalyst, in the present Embodiment, since the fluidized bed and single-stage configuration cracking/reforming reactor 10 is used, the introduction location of the recycle oil is set so that the recycle oil is introduced on the downstream side (the central portion of the reactor 10 as illustrated in FIG. 1) of the introduction location (the bottom portion of the reactor 10) of the feedstock oil into the cracking/reforming reactor 10. Then, the passage of the recycle oil in the cracking/reforming reactor 10 becomes short compared with the passage of the feedstock oil, and therefore the contact time with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 also becomes short.

Meanwhile, the contact times of the respective oils with the catalyst, that is, the retention times are determined by the flow rates of the respective oils per unit time and the lengths of the passages. The flow rates of the respective oils per unit time when only the feedstock oil flows (below the introduction location of the recycle oil) are different from those when the recycle oil flows together with the feedstock oil (above the introduction location of the recycle oil).

<Separation Step>

In the separation step, the product produced in the cracking/reforming reaction step (the cracking/reforming reactor 10) is separated into plural fractions using the separation apparatus 11.

Known distillation apparatuses and gas-liquid separation apparatuses may be used as the separation apparatus 11. An example of the distillation apparatuses may be an apparatus capable of separation of a product by distillation into plural fractions by means of a multistage distillation apparatus such as a stripper. An example of the gas-liquid separation apparatus may be an apparatus including a gas-liquid separation tank; a product inlet pipe through which the product is introduced into the gas-liquid separation tank; a gas component outflow pipe that is provided in the upper part of the gas-liquid separation tank; and a liquid component outflow pipe that is provided in the lower part of the gas-liquid separation tank.

In the separation step (the separation apparatus 11), at least a gas component and a liquid fraction are preferably separated, and the liquid fraction may be further separated into plural fractions. Examples of such a separation step include a form of separating the product into a gas component mainly containing components having 4 or fewer carbon atoms (for example, hydrogen, methane, ethane, and LPG) and a liquid fraction; a form of separating the product into a gas component containing components having 2 or fewer carbon atoms (for example, hydrogen, methane, and ethane) and a liquid fraction; a form of further separating the liquid fraction into LPC a fraction containing monocyclic aromatic hydrocarbons and a heavy fraction; and a form of further separating the liquid fraction into LPG, a fraction containing monocyclic aromatic hydrocarbons, and plural heavy fractions.

<Purification/Recovery Step>

In the purification/recovery step, the monocyclic aromatic hydrocarbons obtained in the separation step (the separation apparatus 11) are purified and collected using the purification/recovery apparatus 12.

In this purification/recovery step, in a case in which a form in which the liquid fraction is not fractionated in the separation step, a step in which a fraction heavier than the monocyclic aromatic hydrocarbons is separated and removed so as to collect monocyclic aromatic hydrocarbons or benzene/toluene/xylene can be applied. In a case in which a fraction heavier than the monocyclic aromatic hydrocarbons is separated in the separation step, a step of recovering benzene/toluene/xylene can be applied. Here, the fraction heavier than monocyclic aromatic hydrocarbons is a heavy fraction having 9 or more carbon atoms, contains polycyclic aromatic hydrocarbons as main components, and, particularly, contains a large amount of naphthalenes and alkylnaphthalenes.

<Heavy Fraction Discharge Step>

In the heavy fraction discharge step, a certain amount of a portion of the heavy fraction having 9 or more carbon atoms obtained from the fraction separated in the separation step (separation apparatus 11) is taken out and discharged out of a system.

In a case in which the heavy fraction discharge step is not included, as the recycle amount increases, low-reactivity components increase in the heavy fraction; however, the present Embodiment includes the heavy fraction discharge step, and discharges a certain amount of the heavy fraction, and therefore it is possible to suppress an increase in the low-reactivity components in the heavy fraction. Therefore, a decrease over time in the yield of the monocyclic aromatic hydrocarbons can be prevented.

The amount of the heavy fraction discharged out of the system is preferably 90 mass % or less of the heavy fraction, more preferably 50 mass % or less, and even more preferably 20 mass % or less. If the amount of the heavy fraction discharged out of the system is set to 90 mass % or less of the heavy fraction, since sufficient recycling is possible, the yield of the monocyclic aromatic hydrocarbons can be further increased.

Heavier hydrocarbons are preferably taken out as the heavy fraction discharged out of the system. For example, since it is relatively difficult to convert fractions containing a large amount of tricyclic aromatic hydrocarbons to monocyclic aromatic hydrocarbons through recycling compared with other fractions a decrease over time in the yield of the monocyclic aromatic hydrocarbons can be prevented by discharging the fractions out of the system. Meanwhile, the heavy fractions discharged out of the system can be used as fuel base materials and the like.

Meanwhile, the heavy fractions may be hydrogenated instead of being discharged.

<Hydrogenation Reaction Step>

In the hydrogenation reaction step, the heavy fraction not discharged out of the system in the heavy fraction discharge step is hydrogenated using the hydrogenation reactor 13. Specifically, the heavy fraction and hydrogen are supplied to the hydrogenation reactor 13, and at least a portion of the polycyclic aromatic hydrocarbons contained in the heavy fraction is hydrogenation-treated using a hydrogenation catalyst.

The polycyclic aromatic hydrocarbons are preferably hydrogenated until the hydrocarbons have an average of one aromatic ring. For example, naphthalene is preferably hydrogenated until it becomes tetraline (naphthenobenzene). If the polycyclic aromatic hydrocarbons are hydrogenated until the hydrocarbons have an average of one aromatic ring or less, the polycyclic aromatic hydrocarbons are easily converted to monocyclic aromatic hydrocarbons when returning to the cracking/reforming reaction step (the cracking/reforming reactor 10).

In addition, since the yield of the monocyclic aromatic hydrocarbons can be further increased, in the hydrogenation reaction step, the content of the polycyclic aromatic hydrocarbons in the hydrogenation reactant of the obtained heavy fraction is preferably set to 40 mass % or less, more preferably set to 25 mass % or less, and even more preferably set to 15 mass % or less. The content of the polycyclic aromatic hydrocarbons in the hydrogenation reactant is preferably smaller than the content of the polycyclic aromatic hydrocarbons in the feedstock oil, and can be decreased by increasing the amount of a hydrogenation catalyst or increasing the reaction pressure. However, it is not necessary to hydrogenate all of the polycyclic aromatic hydrocarbons until the polycyclic aromatic hydrocarbons become saturated hydrocarbons. There is a tendency that excessive hydrogenation brings about an increase in the amount of hydrogen consumption and an increase in the amount of heat generation. Meanwhile, even when the reaction is made to smoothly proceed so that excessive hydrogenation can be suppressed, since the present reaction is an equilibrium reaction, it is not realistic to reduce the polycyclic aromatic hydrocarbons to the equilibrium value or less in operation conditions.

In the present Embodiment, it is also possible to use the by-product of the cracking/reforming reaction step (the cracking/reforming reactor 10) as hydrogen. That is, hydrogen is collected from the gas component obtained in the separation step (the separation apparatus 11) by the hydrogen recovery step (the hydrogen recovery apparatus 14) that will be described below, and the collected hydrogen is supplied to the hydrogenation reaction step (hydrogenation reactor 13) by the hydrogen supply step.

Regarding the reaction type in the hydrogenation reactor 13 (the hydrogenation reaction step), a fixed bed is preferable.

Regarding the hydrogenation catalyst, known hydrogenation catalysts (for example, nickel catalysts, palladium catalysts, nickel-molybdenum-based catalysts, cobalt-molybdenum-based catalysts, nickel-cobalt-molybdenum-based catalysts, and nickel-tungsten-based catalysts) can be used.

The hydrogenation reaction temperature may vary depending on the hydrogenation catalyst used, but the hydrogenation reaction temperature is considered to be usually in the range of 100° C. to 450° C., more preferably 200° C. to 400° C., and even more preferably 250° C. to 380° C.

The hydrogenation reaction pressure may also vary depending on the hydrogenation catalyst or the raw material being used, is preferably set to from 0.7 MPa to 13 MPa, more preferably set to from 1 MPa to 10 MPa, and particularly preferably set to from 1 MPa to 7 MPa. If the hydrogenation reaction pressure is set to 13 MPa or less, a hydrogenation reactor having a low durable pressure can be used, and the facility cost can be reduced.

Meanwhile, the hydrogenation reaction pressure is preferably 0.7 MPa or more in view of the yield of the hydrogenation reaction.

The amount of hydrogen consumption is preferably 3000 scfb (506 Nm³/m³) or less, more preferably 2500 scfb (422 Nm³/m³) or less, and even more preferably 1500 scfb (253 Nm³/m³) or less. On the other hand, the amount of hydrogen consumption is preferably 300 scfb (50 Nm³/m³) or more in view of the yield of the hydrogenation reaction.

The liquid hourly space velocity (LHSV) is preferably set to from 0.1 h⁻¹ to 20 h⁻¹, and more preferably from 0.2 h⁻¹ to 10 h⁻¹. If the LHSV is set to 20 h⁻¹ or less, polycyclic aromatic hydrocarbons can be sufficiently hydrogenated at a lower hydrogenation reaction pressure. On the other hand, when the LHSV is set to 0.1⁻¹ or more, an increase in the scale of the hydrogenation reactor can be avoided.

<Hydrogen Recovery Step>

In the hydrogen recovery step, hydrogen produced as the by-product in the cracking/reforming reaction step (cracking/reforming reactor 10) is collected from the gas component obtained in the separation step (the separation apparatus 11) using the hydrogen recovery apparatus 14.

Regarding the method of recovering hydrogen, there are no particular limitations as long as hydrogen and other gases that are contained in the gas component obtained in the separation step can be separated, and examples thereof include a pressure swing adsorption method (PSA method), a cryogenic separation method, and a membrane separation method. Therefore, an apparatus that collects hydrogen based on the above method (for example, PSA apparatus) is used as the hydrogen recovery apparatus 14.

Ordinarily, the amount of hydrogen collected in the hydrogen recovery step becomes larger than an amount necessary to hydrogenate the heavy fraction.

<Hydrogen Supply Step>

In the hydrogen supply step, hydrogen obtained in the hydrogen recovery step (the hydrogen recovery apparatus 14) is supplied to the hydrogenation reaction step (the hydrogenation reactor 13). The amount of hydrogen supply at that time is regulated depending on the amount of the heavy fraction that is supplied to the hydrogenation reaction step.

When the method includes the hydrogen supply step as in the present Embodiment, the heavy fraction can be hydrogenated using the hydrogen produced as a by-product in the cracking/reforming reaction step (the cracking/reforming reactor 10). A portion or the entirety of the hydrogen supply from an external source can be reduced by preparing a portion or the entire amount of hydrogen used in the present method for producing monocyclic aromatic hydrocarbons from the by-product hydrogen.

<Recycling Step>

In the recycling step, the hydrogenation reactant of the heavy fraction obtained in the hydrogenation reaction step (the hydrogenation reactor 13) is introduced (returned) into the cracking/reforming reactor 10 (the cracking/reforming reaction step) at a location different from the introduction location of the feedstock oil as the recycle oil. That is, the recycle oil is introduced on the downstream side of the introduction location of the feedstock oil into the cracking/reforming reactor 10 so that the time during which the recycle oil (the hydrogenation reactant of the heavy fraction) is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 becomes shorter than the time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10.

By returning the recycle oil to the cracking/reforming reactor 10 (the cracking/reforming reaction step), the heavy fraction, which is a by-product, can also be used as a raw material to obtain monocyclic aromatic hydrocarbons. Therefore, not only can the amount of by-products be reduced, but also the amount of production of monocyclic aromatic hydrocarbons can be increased. Furthermore, since saturated hydrocarbons are also produced by hydrogenation, the hydrogen transfer reaction in the cracking/reforming reaction step can be accelerated. From these, the overall yield of monocyclic aromatic hydrocarbons with respect to the amount of supply of the feedstock oil can be enhanced. Meanwhile, when the heavy fraction is directly returned to the cracking/reforming reactor 10 without being hydrogenation-treated, since the reactivity of polycyclic aromatic hydrocarbons is low, the yield of monocyclic aromatic hydrocarbons is barely increased.

In addition, since the recycle oil is introduced into the cracking/reforming reactor 10 so that the time during which the recycle oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 becomes shorter than the time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10, it is possible to make the recycle oil containing a large amount of components with a high reactivity compared with the feedstock oil appropriately react without being excessively cracking/reforming-reacted in the cracking/reforming reactor 10. Thereby, the yield of monocyclic aromatic hydrocarbons can be increased. In addition, it is possible to suppress an increase in the production amount of light hydrocarbon gas of propane, ethane or the like which results from the excessive cracking/reforming reaction and the consequent frequent occurrence of excessive cracking.

Furthermore, since the retention time of the recycle oil in the cracking/reforming reactor 10 is set to be shorter than the retention time of the feedstock oil, compared with a case in which the retention times are set to be equal, the volume of the cracking/reforming reactor 10 can be decreased. Therefore, apparatus costs or operation costs can be reduced by miniaturizing the cracking/reforming reactor 10.

Embodiment 2

Embodiment 2 of the method for producing monocyclic aromatic hydrocarbons of the invention will be described.

Similarly to Embodiment 1, the method for producing monocyclic aromatic hydrocarbons of the present Embodiment is a method for producing monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms from a feedstock oil (hereinafter, referred to as “monocyclic aromatic hydrocarbons”). The difference of Embodiment 2 from Embodiment 1 is that, while the fluidized bed cracking/reforming reactor 10 is used in the cracking/reforming reaction step in Embodiment 1, as illustrated in FIG. 2, a fixed bed and multistage configuration cracking/reforming reactor 20 is used in Embodiment 2.

In the present Embodiment, since the cracking/reforming reactor 20 employs a fixed bed type configured of two steps of reactors, the cracking/reforming reactor is made up of a former reactor 21 located on the upper stream side and a latter reactor 22 located on the downstream side. Both the former reactor 21 and the latter reactor 22 are loaded with the catalyst for monocyclic aromatic hydrocarbon production described in Embodiment 1 using the fixed bed type. A connection tube 23 is provided between the former reactor 21 and the latter reactor 22. Thereby, a product of a cracking/reforming reaction which is derived from the former reactor 21 is transferred to the latter reactor 22 through the connection tube 23, and, is, furthermore, cracking/reforming-reacted in the latter reactor 22.

Similarly to Embodiment 1, even in the present Embodiment, the recycle oil is returned to the cracking/reforming reaction step (the cracking/reforming reactor 20) using the recycling step. That is, the recycle oil is returned so that the time during which the recycle oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 20 becomes shorter than the time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 20. At that time, in the present Embodiment, the feedstock oil is introduced into the former reactor 21 (the first reactor). In addition, in order to introduce the recycle oil to a reactor on the latter side of the former reactor 21, that is, the latter reactor 22, the recycle oil is supplied to the inside of the connection tube 23 (for example, the inside of a storage tank provided in the connection tube 23).

Thereby, while the feedstock oil comes into contact with the catalyst for monocyclic aromatic hydrocarbon production in both the former reactor 21 and the latter reactor 22, and causes cracking/reforming reactions, the recycle oil comes into contact with the catalyst for monocyclic aromatic hydrocarbon production only in the latter reactor 22, and causes a cracking/reforming reaction.

Therefore, similarly to Embodiment 1, the time during which the recycle oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 20 becomes shorter than the time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 20. As described above, since the recycle oil is introduced into the cracking/reforming reactor 20 so that the contact time of the recycle oil becomes short, it is possible to make the recycle oil appropriately react without being excessively cracking/reforming-reacted in the cracking/reforming reactor 20. Thereby, the yield of monocyclic aromatic hydrocarbons can be increased, and it is possible to suppress an increase in the production amount of gas due to excessive cracking.

Furthermore, since the retention time of the recycle oil in the cracking/reforming reactor 20 is set to be shorter than the retention time of the feedstock oil, apparatus costs or operation costs can be reduced by miniaturizing the cracking/reforming reactor 20.

Meanwhile, in the present Embodiment, the cracking/reforming reactor 20 is configured of two steps, but may be configured of three or more steps. Even in that case, it is preferable to introduce the feedstock oil to the first reactor and to introduce the recycle oil to the reactor on the latter side of the first reactor.

Embodiment 3

Embodiment 3 of the method for producing monocyclic aromatic hydrocarbons of the invention will be described.

The method for producing monocyclic aromatic hydrocarbons of the present Embodiment is a method for producing monocyclic aromatic hydrocarbons from a feedstock oil including the following (i) to (p) steps. In addition, FIG. 3 is a schematic configuration view of a production plant for describing Embodiment 3.

(i) A cracking/reforming reaction step of introducing a feedstock oil into a cracking/reforming reactor 10 loaded with a catalyst for monocyclic aromatic hydrocarbon production, and causing the raw material to react, thereby obtaining a product containing monocyclic aromatic hydrocarbons;

(j) a separation step of separating the product produced in the cracking/reforming reaction step (cracking/reforming reactor 10) into a gas component and a liquid component using the separation apparatus 11;

(k) a hydrogenation reaction step of hydrogenating a portion of the liquid component separated in the separation step (the separation apparatus 11) using the hydrogenation reactor 13;

(l) a hydrogen recovery step of recovering hydrogen produced as a by-product in the cracking/reforming reaction step from the gas component separated in the separation step (separation apparatus 11) using the hydrogen recovery apparatus 14;

(m) a hydrogen supply step of supplying the hydrogen collected in the hydrogen recovery step (hydrogen recovery apparatus 14) to the hydrogenation reaction step (hydrogenation reactor 13).

(n) a purification/recovery step of purifying and recovering a hydrogenation reactant obtained in the hydrogenation reaction step using the purification/recovery apparatus 12;

(o) a heavy fraction discharge step of discharging a portion of the heavy fraction separated and removed from monocyclic aromatic hydrocarbons in the purification/recovery step (the purification/recovery apparatus 12); and

(p) a recycling step of introducing (returning) the heavy fraction not discharged out of a system in the heavy fraction discharge step at a location different from an introduction location of the feedstock oil into the cracking/reforming reactor 10 (cracking/reforming reaction step).

Among the steps (i) to (p), the steps (i), (k), (n), and (p) are essential steps in the invention according to Claim 2, and the steps (j), (l), (m), and (o) are optional steps.

(i) The cracking/reforming reaction step using the cracking/reforming reactor 10 can be carried out similarly to the (a) cracking/reforming reaction step in Embodiment 1.

(j) The separation step using the separation apparatus 11 can be carried out similarly to the (b) separation step in Embodiment 1.

(l) The hydrogen recovery step using the hydrogen recovery apparatus 14 can be carried out similarly to the (f) hydrogen recovery step in Embodiment 1.

(m) The hydrogen supply step can be carried out similarly to the (g) hydrogen supply step in Embodiment 1.

In the (k) hydrogenation reaction step in the present Embodiment, the same hydrogenation catalyst as in the (e) hydrogenation reaction step in Embodiment 1 can be used.

In addition, in the (k) hydrogenation reaction step, unlike the (e) hydrogenation reaction step in Embodiment 1, since the entirety of the liquid component obtained in the separation step is made to flow into the hydrogenation reactor, the obtained monocyclic aromatic hydrocarbons are also hydrogenated. However, the hydrogenation of the monocyclic aromatic hydrocarbons is against the purpose of the present Embodiment. Therefore, in the hydrogen reaction step, the loss amount of monocyclic aromatic hydrocarbons due to hydrogenation is preferably set to 5 mass % or less when the amount of monocyclic aromatic hydrocarbons before the hydrogenation reaction step is designated as 100 mass %. The reaction conditions for achieving the above loss amount are approximately within the range of the reaction conditions of Embodiment 1; however, in order to avoid excessive hydrogenation of monocyclic aromatic hydrocarbons, the temperature is preferably set to be higher than in Embodiment 1.

For example, the hydrogenation reaction temperature also varies depending on the hydrogenation catalyst being used, but is generally set in a range of 250° C. to 450° C., more preferably 300° C. to 400° C., and even more preferably 320° C. to 380° C.

In the (n) purification/recovery step, the monocyclic aromatic hydrocarbons or benzene/toluene/xylene are collected using the purification/recovery apparatus 12, and the heavy fraction is also separated and removed from the monocyclic aromatic hydrocarbons. Here, a fraction heavier than the monocyclic aromatic hydrocarbons is a heavy fraction having 9 or more carbon atoms, and contains the hydrogenation reactant of polycyclic aromatic hydrocarbons and non-hydrated polycyclic aromatic hydrocarbons as the main components.

In the (o) heavy fraction discharge step as well, similarly to the (d) heavy fraction discharge step in Embodiment 1, the amount of the heavy fraction discharged out of the system is preferably 90 mass % or less of the heavy fraction, more preferably 50 mass % or less, and even more preferably 20 mass % or less.

Meanwhile, the heavy fraction may be transferred to the recycling step without being discharged out of the system.

In the (p) recycling step, the hydrogenation reactant of the heavy faction not discharged out of the system is mixed with the feedstock oil, and returned to the cracking/reforming reaction step.

In the present Embodiment as well, the heavy fraction which has been hydrogenated in the hydrogenation reaction step (the hydrogenation reactor 13), and, furthermore, separated from the monocyclic aromatic hydrocarbons in the purification/recovery step (the purification/recovery apparatus 12) is introduced (returned) into the cracking/reforming reactor 10 (the cracking/reforming reaction step) as the recycle oil at a location different from the introduction location of the feedstock oil. That is, similarly to Embodiment 1, the recycle oil is introduced on the downstream side of the introduction location of the feedstock oil into the cracking/reforming reactor 10 so that the time during which the recycle oil (the hydrogenation reactant of the heavy fraction) is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 becomes shorter than the time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10.

Thereby, it is possible to make the recycle oil containing a large amount of components with a high reactivity compared with the feedstock oil appropriately react without being excessively cracking/reforming-reacted in the cracking/reforming reactor 10. Therefore, the yield of monocyclic aromatic hydrocarbons can be increased. In addition, it is possible to suppress an increase in the production amount of light hydrocarbon gas of propane, ethane or the like which results from the excessive cracking/reforming reaction and the consequent frequent occurrence of excessive cracking.

Furthermore, since the retention time of the recycle oil in the cracking/reforming reactor 10 is set to be shorter than the retention time of the feedstock oil, compared with a case in which the retention times are set to be equal, the volume of the cracking/reforming reactor 10 can be decreased. Therefore, apparatus costs or operation costs can be reduced by miniaturizing the cracking/reforming reactor 10.

In addition, since the hydrogenation reactant of the heavy fraction is returned to the cracking/reforming reaction step, the heavy fraction, which is a by-product, can also be used as a raw material to obtain monocyclic aromatic hydrocarbons. Therefore, not only the amount of by-products can be reduced, but also the amount of production of monocyclic aromatic hydrocarbons can be increased. Furthermore, the hydrogen transfer reaction in the cracking/reforming reaction step can also be accelerated. From these, the overall yield of monocyclic aromatic hydrocarbons with respect to the amount of supply of the feedstock oil can be enhanced.

Other Embodiments

The present invention is not intended to be limited to Embodiments 1 to 3, and various modifications can be made to the extent that the gist of the invention is maintained.

For example, in Embodiment 3, the fluidized bed cracking/reforming reactor 10 may be substituted by the fixed bed cracking/reforming reactor 20 in Embodiment 2. In that case, similarly to Embodiment 2, it is preferable to introduce the feedstock oil to the first reactor and to introduce the recycle oil to the reactor on the latter side of the first reactor.

In addition, in Embodiments 1 and 3, the fluidized bed single-stage configuration cracking/reforming reactor is used as the cracking/reforming reactor, but a fluidized bed multistage configuration cracking/reforming reactor may be used. In that case, similarly to the fixed bed cracking/reforming reactor, it is preferable to introduce the feedstock oil to the first reactor and to introduce the recycle oil to the reactor on the latter side of the first reactor.

In addition, the hydrogen used in the hydrogenation reaction steps in the above Embodiments may not be hydrogen which is a by-product of the cracking/reforming reaction step. For example, hydrogen obtained using a known method for producing hydrogen may be used, or hydrogen produced as a by-product in other contact cracking methods may be used.

In addition, in Embodiments 1 and 2, the heavy fraction discharge step may be provided after the hydrogenation reaction step.

EXAMPLES

Examples for producing monocyclic aromatic hydrocarbons (BTX) using the production plants illustrated in FIGS. 1 and 2 will be described below as Examples 1 to 5. In addition, Comparative Examples 1 and 2 and Reference Examples 1 to 8 will also be described below.

Reference Example 1

Recycle oil (the hydrogenation reactant of a heavy fraction) was produced using the production plant illustrated in FIG. 1.

As a feedstock oil, LCO (10 vol % distillation temperature: 224.5° C., 90 vol % distillation temperature: 349.5° C.) indicated in Table 1 was used. In the fluidized bed cracking/reforming reactor 10, as a catalyst for monocyclic aromatic hydrocarbon production, a catalyst containing a binder in MFI type zeolite having 0.2 mass % of gallium and 0.7 mass % of phosphorous supported thereon was used. In addition, a reaction was caused under the conditions adjusted to reaction temperature: 538° C., reaction pressure: 0.3 MPaG, and a time during which nitrogen was introduced, and LCO and the zeolite component contained in the catalyst were brought into contact of 24 seconds, and cracking and modification were carried out. The heavy fraction collected after the cracking and modification was approximately 40 mass % when the feedstock oil was designated as 100 mass %. In addition, the amount of monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) which had been collected after the cracking and modification, and further purified and collected was approximately 36 mass % when the feedstock oil (LCO) was designated as 100 mass %.

Next, the heavy fraction was hydrogenated using a commercially available nickel-molybdenum catalyst under the conditions of a hydrogenation temperature of 350° C., a hydrogenation pressure of 5 M Pa, and a LHSV of 0.5 h⁻¹. The obtained hydrogenation reactants were supplied as recycle oils to Reference Examples 5 to 8 and Examples 1 to 5 that will be described below. The respective conditions and results will be indicated in Table 2.

TABLE 1 Analysis Raw material properties method Density (measurement g/cm³ 0.906 JIS K 2249 temperature 15° C.) Dynamic viscosity mm²/s 3.640 JIS K 2283 (measurement temperature 30° C.) Distillation Initial distillation point ° C. 175.5 JIS K 2254 properties 10 vol % distillation ° C. 224.5 temperature 50 vol % distillation ° C. 274.0 temperature 90 vol % distillation ° C. 349.5 temperature End point ° C. 376.0 Composition Saturated fraction vol % 35 JPI-5S-49 analysis Olefin fraction vol % 8 Whole aromatic vol % 57 fraction Monocyclic aromatic vol % 23 fraction Bicyclic aromatic vol % 25 fraction Tricyclic or vol % 9 higher-cyclic aromatic fraction

TABLE 2 Reference Reference Reference Reference Reference Reference Reference Reference Example Example Example Example Example Example Example Example 1 2 3 4 5 6 7 8 Feedstock oil 100 100 100 100 None None None None (parts by mass) Heavy fraction None None None None 40 40 40 40 (parts by mass) Contact time of 24 12 8 4 feedstock oil [seconds] Contact time of 24 12 8 4 heavy fraction [seconds] Reaction type Fluidized Fluidized Fluidized Fluidized Fluidized Fluidized Fluidized Fluidized bed bed bed bed bed bed bed bed BTX yield [%] 36 33 28 20 50 52 49 47

Reference Examples 2 to 4

The contact times of the feedstock oils with the zeolite component contained in the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor 10 were set to 12 seconds, 8 seconds and 4 seconds. Cracking and modification were carried out in the same manner as in Reference Example 1 except for what has been described above, and production of monocyclic aromatic hydrocarbons was carried out. The amounts of monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) obtained through purification and recovery were 33 mass %, 28 mass %, and 20 mass % respectively when the feedstock oil (LCO) was designated as 100 mass %. The respective conditions and results will be indicated in Table 2.

Reference Examples 5 to 8

The recycle oil (the hydrogenation reactant of the heavy fraction) obtained in Reference Example 1 was cracked and modified in the cracking/reforming reactor 10 under the same conditions as in Reference Example 1 except for the changed contact time. Meanwhile, regarding the supply amount of the recycle oil to the cracking/reforming reactor 10, when the supply amount of the feedstock oil introduced into the cracking/reforming reactor 10 in Reference Example 1 was designated as 100 parts by mass, the supply amount of the recycle oil was 40 parts by mass. The amounts of monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) which had been collected after the cracking and modification, and further purified and collected were 50 mass %, 52 mass %, 49 mass %, and 47 mass % respectively when the recycle oil is designated as 100 mass %. The respective conditions and results will be indicated in Table 2.

Example 1

Monocyclic aromatic hydrocarbons (BTX) were produced using the production plant illustrated in FIG. 1.

As a feedstock oil, LCO (10 vol % distillation temperature: 224.5° C., 90 vol % distillation temperature: 349.5° C.) indicated in Table 1 was used. In addition, as a recycle oil, the hydrogenation reactant of the heavy fraction produced in Reference Example 1 was used.

The supply amount of the feedstock oil introduced into the fluidized bed cracking/reforming reactor 10 was designated as 100 parts by mass, and the supply amount of the recycle oil with respect to the supply amount of the feedstock oil was set to 40 parts by mass. In the cracking/reforming reactor 10, as a catalyst for monocyclic aromatic hydrocarbon production, a catalyst containing a binder in MFI type zeolite having 0.2 mass % of gallium and 0.7 mass % of phosphorous supported thereon was used.

The reaction conditions in the cracking/reforming reactor 10 were adjusted to reaction temperature: 538° C., reaction pressure: 0.3 MPaG, and a time during which nitrogen was introduced, and LCO and the zeolite component contained in the catalyst were brought into contact of 24 seconds. Meanwhile, for the heavy fraction (the recycle oil), the contact time with the catalyst for monocyclic aromatic hydrocarbon production was set to 18 seconds by setting the introduction location into the cracking/reforming reactor 10 on the downstream side of that of the feedstock oil. The respective components were reacted in the above manner, and production of monocyclic aromatic hydrocarbons was carried out.

As a result, the amount (BTX yield) of the obtained monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) was 40 mass % when the total amount of the feedstock oil (LCO) and the recycle oil was designated as 100 mass %. The respective conditions and results will be indicated in Table 3.

TABLE 3 Comparative Comparative Example 1 Example 2 Example 3 Example 4 Example 5 Example 1 Example 2 Feedstock oil 100 100 100 100 100 100 100 (parts by mass) Heavy fraction 40 40 40 40 40 40 40 (parts by mass) Contact time of 24 24 24 24 8 24 8 feedstock oil [seconds] Contact time of 18 12 8 4 3 24 8 heavy fraction [seconds] Reaction type Fluidized bed Fluidized bed Fluidized bed Fluidized bed Fixed bed Fluidized bed Fixed bed BTX yield [%] 40 42 40 39 38 39 37 Yield of gas [%] — — — 9 — 11 — Volume of 91 86 80 76 — 100 — catalyst layer

Example 2

As the reaction condition in the cracking/reforming reactor 10, the contact time of the heavy fraction (the recycle oil) with the zeolite component contained in the catalyst for monocyclic aromatic hydrocarbon production was set to 12 seconds, and a reaction was caused. Production of monocyclic aromatic hydrocarbons was carried out in the same manner as in Example 1 except for what has been described above.

As a result, the amount (BTX yield) of the obtained monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) was 42 mass % when the total amount of the feedstock oil (LCO) and the recycle oil was designated as 100 mass %. The respective conditions and results will be indicated in Table 3.

Example 3

As the reaction condition in the cracking/reforming reactor 10, the contact time of the heavy fraction (the recycle oil) with the zeolite component contained in the catalyst for monocyclic aromatic hydrocarbon production was set to 8 seconds, and a reaction was caused. Production of monocyclic aromatic hydrocarbons was carried out in the same manner as in Example 1 except for what has been described above.

As a result, the amount (BTX yield) of the obtained monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) was 40 mass % when the total amount of the feedstock oil (LCO) and the recycle oil was designated as 100 mass %. The respective conditions and results will be indicated in Table 3.

Example 4

As the reaction condition in the cracking/reforming reactor 10, the contact time of the heavy fraction (the recycle oil) with the zeolite component contained in the catalyst for monocyclic aromatic hydrocarbon production was set to 4 seconds, and a reaction was caused. Production of monocyclic aromatic hydrocarbons was carried out in the same manner as in Example 1 except for what has been described above.

As a result, the amount (BTX yield) of the obtained monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) was 40 mass % when the total amount of the feedstock oil (LCO) and the recycle oil was designated as 100 mass %. In addition, the yield of gas obtained using the hydrogen recovery apparatus 14 in the production plant in FIG. 1 was found to be 9 mass %. The respective conditions and results will be indicated in Table 3.

Example 5

Monocyclic aromatic hydrocarbons (BTX) were produced using the production plant illustrated in FIG. 2.

As a feedstock oil, LCO indicated in Table 1 was used. In addition, as a recycle oil, the heavy fraction derived from the hydrogenation reactor 13 was used. The supply amount of the feedstock oil introduced into the fixed bed cracking/reforming reactor 20 was designated as 100 parts by mass, and the supply amount of the recycle oil with respect to the supply amount of the feedstock oil was set to 40 parts by mass. In the cracking/reforming reactor 20, as a catalyst for monocyclic aromatic hydrocarbon production, a catalyst containing a binder in MR type zeolite having 0.2 mass % of gallium and 0.7 mass % of phosphorous supported thereon was used.

The reaction conditions in the cracking/reforming reactor 20 were adjusted to reaction temperature: 538° C., reaction pressure: 0.3 MPaG, and a time during which nitrogen was introduced, and LCO and the zeolite component contained in the catalyst were brought into contact of 8 seconds for both the former reactor 21 and the latter reactor 22. Meanwhile, for the heavy fraction (the recycle oil), the contact time with the zeolite component included in the catalyst for monocyclic aromatic hydrocarbon production was set to 3 seconds by setting the introduction location into the cracking/reforming reactor 20 at the connection tube 23, that is, the latter reactor 22. The respective components were reacted in the above manner, and production of monocyclic aromatic hydrocarbons was carried out.

As a result, the amount (BTX yield) of the obtained monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) was 38 mass % when the total amount of the feedstock oil (LCO) and the recycle oil was designated as 100 mass %. The respective conditions and results will be indicated in Table 3.

Comparative Example 1

Monocyclic aromatic hydrocarbons (BTX) were produced using the production plant illustrated in FIG. 1.

The introduction location of the heavy fraction (the recycle oil) into the cracking/reforming reactor 10 was set to the same location as that of the feedstock oil, whereby the contact time of the heavy fraction (the recycle oil) with the zeolite component included in the catalyst for monocyclic aromatic hydrocarbon production was set to 24 seconds, similarly to the contact time of the feedstock oil. Production of monocyclic aromatic hydrocarbons was carried out in the same manner as in Example 1 except for what has been described above.

As a result, the amount (BTX yield) of the obtained monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) was 39 mass % when the total amount of the feedstock oil (LCO) and the recycle oil was designated as 100 mass %. In addition, the yield of gas obtained using the hydrogen recovery apparatus 14 in the production plant in FIG. 1 was found to be 11 mass %. The respective conditions and results will be indicated in Table 3.

Comparative Example 2

Monocyclic aromatic hydrocarbons (BTX) were produced using the production plant illustrated in FIG. 2.

The contact time of the feedstock oil with the zeolite component contained in the catalyst for monocyclic aromatic hydrocarbon production was set to 8 seconds. In addition, the introduction location of the heavy fraction (the recycle oil) into the cracking/reforming reactor 20 was set to the same location as that of the feedstock oil, whereby the contact time of the heavy fraction (the recycle oil) with the zeolite component included in the catalyst for monocyclic aromatic hydrocarbon production was set to 8 seconds, similarly to the contact time of the feedstock oil.

Production of monocyclic aromatic hydrocarbons was carried out in the same manner as in Example 5 except for what has been described above.

As a result, the amount (BTX yield) of the obtained monocyclic aromatic hydrocarbons (benzene, toluene, and xylene) was 37 mass % when the total amount of the feedstock oil (LCO) and the recycle oil was designated as 100 mass %. The respective conditions and results will be indicated in Table 3.

From the above results, the following facts were found.

In Examples 1 to 4 in which the contact times of the recycle oils (the heavy fractions) were set to be shorter than those of the raw materials oil in the fluidized beds, the yields of BTX were equal to or larger than that in Comparative Example 1 in which both contact times were set to be equal. In addition, in Example 5 in which the contact time of the recycle oil (the heavy fraction) was set to be shorter than that of the feedstock oil in the fluidized bed, a larger BTX yield was obtained than in Comparative Example 2 in which both contact times were set to be equal.

In Examples 1 to 4, the contact times of the recycle oils (the heavy fractions), that is, the retention times in the cracking/reforming reactors became shorter than in Comparative Example 1. Therefore, as a result of obtaining the necessary volume of the cracking/reforming reactor based on the retention times (contact times) of the recycle oils (the heavy fractions) through computation, as illustrated in Table 3, the volumes were 91 in Example 1, 86 in Example 2, 80 in Example 3, and 76 in Example 4 in a case in which the volume in Comparative Example 1 was considered to be 100. Therefore, as the retention time (contact time) of the recycle oil became shorter, the necessary volume of the cracking/reforming reactor became smaller, whereby it was possible to miniaturize the cracking/reforming reactors.

In Comparative Example 4, the yield of gas was low compared with in Comparative Example 1, and therefore excessive cracking was suppressed compared with Comparative Example 1.

INDUSTRIAL APPLICABILITY

The invention is useful for production of monocyclic aromatic hydrocarbons in which LCO obtained from an FCC apparatus and kerosene, gas oil or the like obtained from a crude oil distillation apparatus are used as raw materials.

REFERENCE SIGNS LIST

-   -   10 CRACKING/REFORMING REACTOR     -   11 SEPARATION APPARATUS     -   12 PURIFICATION/RECOVERY APPARATUS     -   13 HYDROGENATION REACTOR     -   14 HYDROGEN RECOVERY APPARATUS     -   20 CRACKING/REFORMING REACTOR     -   21 FORMER REACTOR     -   22 LATTER REACTOR     -   23 CONNECTION TUBE 

1. A method for producing monocyclic aromatic hydrocarbons, by which monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms are produced from a feedstock oil having a 10 vol % distillation temperature of 140° C. or higher and a 90 vol % distillation temperature of 380° C. or lower, comprising: a cracking/reforming reaction step of introducing the feedstock oil into a cracking/reforming reactor loaded with a catalyst for monocyclic aromatic hydrocarbon production containing a crystalline aluminosilicate, bringing the feedstock oil into contact with the catalyst for monocyclic aromatic hydrocarbon production, and causing the feedstock oil to react, thereby obtaining a product containing monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms; a purification/recovery step of purifying and recovering the monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms separated from the product produced in the cracking/reforming reaction step; a hydrogenation reaction step of hydrogenating a heavy fraction having 9 or more carbon atoms separated from the product produced in the cracking/reforming reaction step; and a recycling step of returning a hydrogenation reactant of the heavy fraction obtained in the hydrogenation reaction step to the cracking/reforming reaction step, wherein, in the recycling step, the hydrogenation reactant of the heavy fraction is introduced at a location different from an introduction location of the feedstock oil into the cracking/reforming reactor so that a time during which the hydrogenation reactant of the heavy fraction is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor becomes shorter than a time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor.
 2. A method for producing monocyclic aromatic hydrocarbons, by which monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms are produced from a feedstock oil having a 10 vol % distillation temperature of 140° C. or higher and a 90 vol % distillation temperature of 380° C. or lower, comprising: a cracking/reforming reaction step of introducing the feedstock oil into a cracking/reforming reactor loaded with a catalyst for monocyclic aromatic hydrocarbon production containing a crystalline aluminosilicate, bringing the feedstock oil into contact with the catalyst for monocyclic aromatic hydrocarbon production, and causing the feedstock oil to react, thereby obtaining a product containing monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms; a hydrogenation reaction step of hydrogenating a portion of the product produced in the cracking/reforming reaction step; a purification/recovery step of distilling a hydrogenation reactant obtained in the hydrogenation reaction step so as to purify and collect monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms; and a recycling step of returning the heavy fraction having 9 or more carbon atoms separated and removed from the monocyclic aromatic hydrocarbons having 6 to 8 carbon atoms in the purification/recovery step, wherein, in the recycling step, the heavy fraction is introduced at a location different from an introduction location of the feedstock oil into the cracking/reforming reactor so that a time during which the hydrogenation reactant of the heavy fraction is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor becomes shorter than a time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor.
 3. The method for producing monocyclic aromatic hydrocarbons according to claim 1, wherein, in the recycling step, when the time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor is considered to be 100%, the hydrogenation reactant of the heavy fraction or the heavy fraction is introduced at a location different from the introduction location of the feedstock oil into the cracking/reforming reactor so that the time during which the hydrogenation reactant of the heavy fraction or the heavy fraction is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor becomes 10% to 80%.
 4. The method for producing monocyclic aromatic hydrocarbons according to claim 1, wherein, in a case in which the cracking/reforming reactor employs a fluidized bed type and a single-stage configuration, the hydrogenation reactant of the heavy fraction or the heavy fraction is introduced on a downstream side of the introduction location of the feedstock oil into the cracking/reforming reactor.
 5. The method for producing monocyclic aromatic hydrocarbons according to claim 1, wherein, in a case in which the cracking/reforming reactor employs a fixed bed type and a multistage configuration, the feedstock oil is introduced into a first reactor in the cracking/reforming reactor, and the hydrogenation reactant of the heavy fraction or the heavy fraction is introduced into a reactor behind the first reactor.
 6. The method for producing monocyclic aromatic hydrocarbons according to claim 1, wherein the crystalline aluminosilicate contained in the catalyst for monocyclic aromatic hydrocarbon production used in the cracking/reforming reaction step contains a medium-pore zeolite and/or a large-pore zeolite as a main component.
 7. The method for producing monocyclic aromatic hydrocarbons according to claim 1, comprising: a hydrogen recovery step of recovering hydrogen produced as a by-product in the cracking/reforming reaction step from a product obtained in the cracking/reforming reaction step; and a hydrogen supply step of supplying the hydrogen collected in the hydrogen recovery step to the hydrogenation reaction step.
 8. The method for producing monocyclic aromatic hydrocarbons according to claim 2, wherein, in the recycling step, when the time during which the feedstock oil is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor is considered to be 100%, the hydrogenation reactant of the heavy fraction or the heavy fraction is introduced at a location different from the introduction location of the feedstock oil into the cracking/reforming reactor so that the time during which the hydrogenation reactant of the heavy fraction or the heavy fraction is in contact with the catalyst for monocyclic aromatic hydrocarbon production in the cracking/reforming reactor becomes 10% to 80%.
 9. The method for producing monocyclic aromatic hydrocarbons according to claim 2, wherein, in a case in which the cracking/reforming reactor employs a fluidized bed type and a single-stage configuration, the hydrogenation reactant of the heavy fraction or the heavy fraction is introduced on a downstream side of the introduction location of the feedstock oil into the cracking/reforming reactor.
 10. The method for producing monocyclic aromatic hydrocarbons according to claim 2, wherein, in a case in which the cracking/reforming reactor employs a fixed bed type and a multistage configuration, the feedstock oil is introduced into a first reactor in the cracking/reforming reactor, and the hydrogenation reactant of the heavy fraction or the heavy fraction is introduced into a reactor behind the first reactor.
 11. The method for producing monocyclic aromatic hydrocarbons according to claim 2, wherein the crystalline aluminosilicate contained in the catalyst for monocyclic aromatic hydrocarbon production used in the cracking/reforming reaction step contains a medium-pore zeolite and/or a large-pore zeolite as a main component.
 12. The method for producing monocyclic aromatic hydrocarbons according to claim 2, comprising: a hydrogen recovery step of recovering hydrogen produced as a by-product in the cracking/reforming reaction step from a product obtained in the cracking/reforming reaction step; and a hydrogen supply step of supplying the hydrogen collected in the hydrogen recovery step to the hydrogenation reaction step. 